The present invention relates to a heat-resistant and/or fire-retardant synthetic fiber. More particularly, the present invention relates to a polyethylene fiber excellent in heat resistance and/or fire retardance. The present invention also relates to a process for preparing such fiber.
Polyethylene fiber is inexpensive and not only has excellent strength properties which particularly are unchanged even in water but also has various excellent properties such as resistance to acids, resistance to alkalis and resistance to organic solvents so that it is widely used as industrial materials such as a filter fabric, and further it is also widely utilized as a fishing net because it is excellent in water resistance and highly stable for use in seawater. However, polyethylene fiber is limited in its use since it undergoes heat distortion such as heat shrinkage, etc. when heated at 70.degree. C., melts at 110.degree. to 120.degree. C. and loses the form of fiber entirely at 140.degree. C. Further, polyethylene fiber is hydrophobic as a property common to synthetic fibers and is difficult to dye, and it also has the deficiency of easily causing frictional static electrification.
If the stability of polyethylene to heating is improved without damaging its excellent mechanical properties, the use of polyethylene fiber will be enlarged by this alone. It has been known to irradiate polyethylene with an ionizing radiation such as .gamma.-rays and electron beams with 5 to 50 Mrad to form a cross linkage for the purpose of improving the heat stability of polyethylene. This method is effective for preventing polyethylene from melting at its melting point (135.degree. to 140.degree. C.), but the method is unsatisfactory for preventing the heat distortion of fibrous polyethylene. A polyethylene fiber easily catches fire, melts and burns on contact with a flame since the fiber is a high molecular weight material composed of carbon and hydrogen only. In recent years from a viewpoint of safety, a problem to be urgently solved is to make fiber materials hard to burn. If it becomes possible to make a polyethylene fiber hard to burn, the range of use will be remarkably enlarged.
It has hitherto been known to add and blend a halogenated compound, a phosphorus-containing compound or antimony oxide or to react with a compound containing a halogen or phosphorus to make a high molecular weight material such as polyethylene hard to burn. A halogen, phosphorus, antimony, etc. are known as elements making high molecular material hard to burn.
As a result of research on the stability of polyethylene fiber to heating, the present inventors have found that, when graft polymerizing polyethylene fiber with acrylic acid or, after graft polymerization, by converting it to a salt of a mono-valent metal such as sodium, potassium, lithium, etc., a salt of a divalent metal such as calcium, zinc. etc. or a salt of a trivalent metal such as aluminum, etc., if the graft percent is more than 15% the grafted polyethylene fiber becomes fire retardant, and if the graft percent is more than 20% heat shrinkage barely occurs and, in particular, if the graft percent is more than 30% an improved polyethylene fiber having excellent mechanical properties which is not melt broken even by heating to 300.degree. C. can be obtained.